1. Field of the Invention
The present invention relates to a method of manufacturing a bonded substrate, a bonded substrate manufactured thereby, a method of manufacturing a solid-state imaging apparatus, a solid-state imaging apparatus manufactured thereby, and a camera. More particularly, the invention relates to a method of manufacturing a solid-state imaging apparatus in which pixels having photodiodes are arranged in a matrix pattern on a light-receiving surface, a solid-state imaging apparatus manufactured thereby, a method of manufacturing a bonded substrate for the same, a bonded substrate manufactured thereby, and a camera including the solid-state imaging apparatus.
2. Description of the Related Art
A method for directly bonding semiconductor substrates to each other, generally, uses bonding by dehydration condensation between silicon oxide films, as represented by a method of manufacturing a SOI (Silicon On Insulator) type substrate.
In the method of manufacturing the SOI substrate, for example, after tentative bonding of silicon oxide films to each other, a heat treatment at a high temperature is conducted to effect dehydration condensation, thereby enhancing the bonding strength. In order to obtain a sufficient bonding strength, a sufficient dehydration condensation reaction may be needed and, in general, a high temperature of 800 to 1200° C. may be necessary.
While a heat treatment at a lower temperature has also been reported in this connection, temperatures in excess of 500° C. have been reported, as described in, for example, Japanese Patent Laid-open No. 2001-274368 (hereinafter referred to as Patent Document 1).
For conducting bonding at a further lower temperature, there has been carried out a method of utilizing acceleration of modification of a silicon oxide film surface by irradiation with plasma or by treatment with a liquid chemical.
In addition, a technique frequently used as a method for bonding at a low temperature is bonding by an adhesive, as described in, for example, Japanese Patent Laid-open No. 2008-300551 (hereinafter referred to as Patent Document 2).
In the case where semiconductor substrates which carry semiconductor devices with metallic wiring and the like are directly bonded to each other for manufacturing a semiconductor apparatus, the upper limit of a treating temperature in a heat treatment applied to the substrates is limited by the melting point of the metal used for the wiring and by the characteristics of the semiconductor apparatus desired.
Particularly, in the case where copper is used for wiring, the upper limit of the heat treatment temperature should be not more than 400° C.
In this case, as a method of bonding semiconductor substrates to each other at a low temperature of 400° C. or below, the above-mentioned bonding through surface modification by irradiation with plasma or by treatment with a liquid chemical or the above-mentioned bonding by an adhesive is normally carried out.
The method based on the use of an adhesive has problems in that the upper limit of temperature is determined by the heat-resistant temperature of the adhesive itself or that durability to liquid chemicals used in forming the semiconductor devices is low.
On the other hand, in the case of bonding through surface modification by irradiation with plasma or by treatment with a liquid chemical, a treatment at a low temperature of 400° C. or below is insufficient in temperature for the dehydration condensation intended, so that the bonding strength obtained thereby is lower than the bonding strength obtained by a heat treatment at a high temperature of not less than 800° C.
The high bonding strength obtained by the high-temperature heat treatment is considered to be attributable partly to a phenomenon in which hydroxyl groups are vibrated by thermal vibration, whereby intervals between the hydroxyl groups are shortened, resulting in that even those hydroxyl groups which are spaced too far from each other at a low temperature to react with each other can be brought into dehydration condensation.
In the case of a low bonding strength, there is a fear that exfoliation from the faying surface (bonding surface or joint surface) might occur during a processing of the wafer after bonding. Even in the cases of low-temperature bonding, therefore, there is a need for a technology by which a bonding strength as high as possible can be obtained.
In order to raise the bonding strength obtained by a low-temperature heat treatment, it is important to increase the density of Si—O—Si bonds produced by the dehydration condensation reaction.
Examples of the means for increasing the Si—O—Si bond density include:
(1) to apply such an energy that vibration comparable to the thermal vibration of hydroxyl groups generated at high temperature can be obtained, to the interface after tentative bonding; and
(2) to form such a faying surface that a hydroxyl group interval permitting a dehydration condensation reaction even in the case of small amplitude of vibration can be obtained, specifically, to raise the density of hydroxyl groups in the surface.
As the approach of (1) above, there may be contemplated a method of irradiating with optical energy, such as laser, that acts on the faying surface. In the case where semiconductor devices are present, however, a reaction should be induced only at the faying surface.
In order to meet the requirement, it may be necessary to provide a special light-absorbing layer at the faying surface and to use a substrate transparent to the wavelength of the light as at least one of the substrates to be bonded. Such measures are highly difficult to realize in an ordinary fabrication process for semiconductor devices.
The method of (2) above, on the other hand, can be embodied by regulations as to the surface modification process or as to the film properties at the faying surface, and is therefore easier to realize.
As the method based on regulations of the film properties at the faying surface, a large number of methods have been attempted. For instance, Japanese Patent Laid-open No. 2009-173949 (hereinafter referred to as Patent Document 3) discloses a method in which a functional group is introduced into an oxide film and eliminated (leaving) portions of the functional groups are utilized for bonding.
Though formation of hydroxyl groups is easily achieved by this method, however, the density of the hydroxyl groups formed at the eliminated (leaving) portions of the functional groups is the same as the density of the hydroxyl groups formed at the surface of the oxide film, so that the hydroxyl groups do not contribute to enhancement of the bonding strength obtained by a low-temperature heat treatment.
In addition, most of functional groups are hydrophobic in nature. Therefore, there arises a problem that the uneliminated (unleaving) functional groups in the oxide film repel the water molecules produced by dehydration condensation, resulting in easy formation of voids at the faying surface.
Japanese Patent Laid-open No. 2005-285988 (hereinafter referred to as Patent Document 4) contains a description of a back-illuminated type solid-state imaging apparatus.
A. Kuwabara et al., Phys. Rev., B78, (2008) 064104 (hereinafter referred to as Non-Patent Document 1) describes a mean inter-atomic distance in such material as silicon oxide, silicon nitride and silicon carbide.
R. H. Dauskardt et al., Engineering Fracture Mechanics 61, (1998) pp. 141 to 162 (hereinafter referred to as Non-Patent Document 2) describes a bonding strength measuring method based on the four-point bending technique.